Density gradient centrifugation is one of the most widely used separation methods in biochemical and biological sciences. Fractionation methods which have been developed usually involve displacement of the entire gradient either up or down in the centrifuge tube. One of the earliest and simplest of these methods of fractionation was one in which the bottom of the centrifuge tube is pierced with a fine needle and then the drops are collected. Recent improvements of this technique involve controlling the flow rate of the effluent or the speeds at which air or distilled water enters at the top of the centrifuge tube. A second common method of collection involves producing a dense solution at the bottom of the centrifuge tube and then allowing the gradient to flow up into an inverted collection funnel at the top.
These two fractionation techniques, however, share several disadvantages. First, vertical movement of the gradient causes contamination of some of the fractions of the gradient with particles being retarded near the wall. Furthermore, these methods also lack the important advantage of allowing isolation of individual bands made visible by scattered light, since such bands are lost from view as they near the collection point.
Two prior art methods which do permit the isolation of the individual bands include inserting a needle through the centrifuged tube just below the band, or lowering a capillary tube into the gradient until the open end lies just below the band. Both of these methods, however, can delute and contaminate the sample because collection is made from a single small opening, and is inadequate for isolating numerous bands from the same gradient.